1. Field of the Invention
The present invention relates to a method and apparatus for reproducing data from a signal read out from a recording medium or transferred by a communication means and, more particularly, to a method and apparatus for reproducing data subjected to convolution based on partial response, by using maximum likelihood.
2. Related Background Art
In recent years, high-density information recording or information transfer at a high transfer rate has become popular for a data reproducing apparatus. For example, a signal processing method called PRML is known as a means for realizing such high-density recording or high transfer rate. The PRML signal processing method uses a combination of PR (Partial Response) and ML (Maximum Likelihood). Partial response is a technique of reproducing data by waveform equalization. A signal read out from a recording medium or transferred by a communication means is output while its waveform is distorted by intercode interference. To properly reproduce data from this signal, the noise component may be removed by filtering. However, the frequency band of the noise component overlaps that of the data signal component over a wide range in fact, and it is therefore difficult to remove the noise component. In partial response, data convolution is performed on the basis of predetermined rules to narrow the band of the data signal, thereby performing data reproduction with a high S/N ratio. The use of partial response allows high-density signal recording in a recording medium and also can cope with an increase in influence of intercode interference on the signal read out from the medium, so that the capacity of information recordable in the recording medium can be increased.
Several schemes are known as partial response, and the above scheme is normally expressed as "PR(1,1)". "PR" is the initials of "partial response". The first "1" in the parentheses represents the original signal, and the second "1" represents that a signal delayed by one bit is superposed on the original signal. Assume a binary signal "010100101100", as shown in FIG. 14A. A signal shown in FIG. 14B, which is delayed by one bit, is superposed on the binary signal, thereby obtaining a signal shown in FIG. 14C. The signal shown in FIG. 14C is called a signal obtained upon waveform equalization by PR(1,1). This signal is a ternary signal with signal amplitude levels "1", "0", and "-1". The signal shown in FIG. 14C has a frequency band narrower than that of the original signal.
Similarly, PR(1,2,1) represents that a signal (FIG. 15B) obtained by delaying a binary signal as shown in FIG. 15A by one bit and doubling its amplitude and a signal (FIG. 15D) obtained by delaying the signal (FIG. 15A) by two bits but with the same amplitude as that of the original signal (FIG. 15A) are superposed on the original binary signal. Consequently, a signal shown in FIG. 15D is obtained. The signal shown in FIG. 15D is called a signal obtained upon waveform equalization by PR(1,2,1). This signal is a quinary signal with signal amplitude levels "2", "1", "0", "-1", and "-2". The signal shown in FIG. 15D has a frequency band narrower than that of the original signal. It can be regarded in PR(1,2,1) that signals obtained upon waveform equalization by PR(1,1) are shifted from each other by one bit, and convolution is further performed.
Maximum likelihood is a technique used when data is to be detected in accordance with information convolution by the above-described partial response. Data with the highest probability is selected to determine the data.
For a signal obtained upon waveform equalization by partial response, a value at a certain time point contains previous information, e.g., information two bits before in PR (1,2,1). For this reason, the original data cannot be determined on the basis of only that value. When maximum likelihood is performed, the data can be properly reproduced from the signal obtained by partial response.
As a specific technique of realizing such maximum likelihood, a matrix method is proposed in, e.g., "A Reliable Signal Detection Method for the Combination of PRML Method and Ternary Recording Code", S. Tazaki, et. al., Proc. '94 IEEE ISIT, p. 214 (June, 1994). The present inventors have filed on an information recording/reproducing apparatus which employs partial response PR(1,1) and reproduces binary digital information by maximum likelihood, which is disclosed in Japanese Patent Application No. 6-181363.
In the former technique, however, a large usable memory must be ensured, and the calculation time is long. In the latter apparatus, detection data is input to a shift register array, and signal processing performed. Data "1" and "0" stored in the shift register correspond to the number of states, so this apparatus can hardly cope with four or more states.